A general direct injection type engine is able to switch its combustion mode between a lean combustion mode in which the engine operates at a lean air/fuel ratio to achieve homogeneous combustion, and a stoichiometric combustion mode in which the engine operates at the stoichiometric air/fuel ratio to achieve homogenous combustion. In this case, a controller for the engine stores a combustion mode map used for setting the combustion mode based on the engine speed and the engine load, and controls switching between the lean combustion mode and the stoichiometric combustion mode, depending on the engine operating conditions.
When the engine capable of switching the combustion mode is controlled to change the air/fuel ratio between a lean air/fuel ratio and the stoichiometric air/fuel ratio in accordance with the operating conditions, the engine produces different amounts of torque if the air/fuel ratio is largely changed, and a large stepped change (e.g., an abrupt reduction) in torque may occur at the time of change of the air/fuel ratio, resulting in deterioration of the driveability.
In order to prevent the occurrence of a stepped change in torque at the time of change of the air/fuel ratio, it has been proposed to perform control for retarding the ignition timing. For example, when the combustion mode is switched from the stoichiometric combustion mode to the lean combustion mode, the controller controls operation of the engine at the stoichiometric air/fuel ratio during the switching control, and then changes the air/fuel ratio from the stoichiometric air/fuel ratio to a lean air/fuel ratio after the switching control. More specifically, when a command for switching from the stoichiometric combustion mode to the lean combustion mode is issued in response to the current engine operating conditions, the controller increases the throttle opening so as to increase the intake air amount, and also increases the fuel injection amount, thereby to keep the engine operating at the stoichiometric air/fuel ratio. During this operation, the controller retards the ignition timing so as to restrict or inhibit an increase of torque produced by the engine. After a lapse of a predetermined period of time, the controller advances the ignition timing so as to establish the ignition timing suitable for the engine operating conditions, and reduces the fuel injection amount so as to change the air/fuel ratio from the stoichiometric ratio to a lean air/fuel ratio. An example of the internal combustion engine controlled in this manner is disclosed in Japanese Patent Application Publication No. 08-114166 (JP-A-08-114166).
In the case where the engine is equipped with a turbo-supercharger or turbocharger, however, the boost pressure rises to a level higher than necessary due to an increase of exhaust energy when the combustion mode is switched between a non-supercharged stoichiometric combustion mode and a supercharged lean combustion mode, and a stepped change (e.g., an abrupt reduction) in torque occurs when the control for changing the air/fuel ratio is finished, resulting in deterioration of the driveability. More specifically, when the combustion mode of the engine switches from the non-supercharged stoichiometric combustion mode to the supercharged lean combustion mode, the exhaust gas temperature rises due to retardation of the ignition timing, and the actual boost pressure produced by the turbo-supercharger rises to a level equal to or higher than a target boost pressure. As the boost pressure increases more than necessary, the torque increases, and a stepped change (e.g., an abrupt reduction) in torque occurs since it is necessary to reduce the increased torque when the control for changing the air/fuel ratio is finished. In this case, as the engine load (or the intake air amount) increases, the amount of reduction of torque relative to the retard amount of the ignition timing increases, and therefore, an apparently large stepped change in torque occurs at the time of switching of the combustion mode, resulting in deterioration of the combustion mode.